This invention relates to an efficient and cost-effective system and process for argon purification, particularly a pressure swing adsorption (PSA) process for argon purification of a feed from a cryogenic air separation plant.
In the operation of a cryogenic air separation plant to produce oxygen and nitrogen, there may be recovered a crude argon stream containing about 1-5% by volume oxygen and about 1% by volume nitrogen. To remove oxygen and nitrogen to further purify such crude argon stream, one of the methods generally employed is the reaction of oxygen with excess hydrogen. Such a process is energy intensive, since the gas stream is heated to a high reaction temperature and later cooled to cryogenic temperatures to remove the excess hydrogen and residual nitrogen. In addition, such a process may not be practical in those parts of the world where hydrogen availability and shipment are limited. Another method, based on cryogenic distillation only, requires the use of a large or superstaged argon column due to the small difference in the relative volatility between argon and oxygen. Additional methods include the use of solid electrolyte membrane(s), two beds in series vacuum pressure swing adsorption (VPSA) process, getter/deoxo system, and temperature swing adsorption (TSA).
The cryogenic rectification of air to produce oxygen, nitrogen and/or argon is well known in prior art processes. Typically, a three stage cryogenic process is used, wherein, feed air is separated into nitrogen and oxygen in a double column system that uses nitrogen top vapor from a higher pressure column to reboil oxygen-rich bottom liquid in a lower pressure column, and argon-containing fluid from the lower pressure column is passed into an argon side arm column for the production of argon product. For example, U.S. Pat. No. 5,440,884 by Bonaquist and Lockett, disclosed a three stage cryogenic rectification system, employing a double column system with an associated argon side arm column, to produce high purity ( greater than 99.999%) argon. In order to produce high purity argon, a large or superstaged argon column was used. According to U.S. Pat. No. 5,440,884, the large argon column is preferably divided into two separate argon columns, and a stripping column is used upstream of the double main column to suppress the thermodynamic irreversibility of the argon column top condenser and the lower pressure column.
More recently, Jain et al., U.S. Pat. No. 5,601,634 discloses a cryogenic temperature swing adsorption process to produce high purity argon from a two-phase liquid-vapor mixture. According to their invention, each adsorbent bed contains one or more adsorbents selective for nitrogen and/or oxygen at a temperature between the bubble point and the dew point of the two phase mixture. In the most efficient embodiment of that invention, each adsorbent bed contains a nitrogen adsorbent layer that precedes an oxygen adsorbent layer. In addition, according to that invention, the nitrogen selective adsorbent is zeolite type X or mordenite, and the oxygen selective adsorbent layer is carbon molecular sieve or 4A zeolite.
Kovak et al., U.S. Pat. No. 5,159,816, discloses the production of high purity argon (less than 5 ppm each of oxygen and nitrogen) by cryogenic adsorption wherein a crude argon stream flows through a bed of adsorbent that preferentially adsorbs nitrogen, and then through another bed that preferentially adsorbs oxygen. The process is conducted without the need of refrigeration by maintaining a low gas space velocity through the beds, and by limiting the oxygen and nitrogen in the feed (crude argon) to less than 0.8 mole percent and 0.5 mole percent, respectively.
Also, Bligh et al., U.S. Pat. No. 4,239,509, disclosed a low temperature (xe2x88x92250xc2x0 F.) method for purifying crude argon via temperature swing adsorption (TSA) wherein a feed mixture containing argon, oxygen and nitrogen goes through the steps of reducing the amount of nitrogen to trace level ( less than 0.15% by volume) by passing the crude argon through a first bed of 5A and/or 13X molecular sieves surrounding and in thermal contact with a second bed of 4A zeolite with a wall separating the 5A and/or 13X zeolite from said 4A zeolite, and passing the remaining oxygen and argon, together with residual nitrogen, through the second bed to produce high purity argon. After the adsorption step at xe2x88x92250xc2x0 F. is completed, nitrogen at 180xc2x0 F. is used as a purge gas to regenerate the adsorption bed(s).
However, the aforementioned processes suffer from low yield and low purity of the argon product. Another prior art process includes, for example, U.S. Pat. No. 4,477,265 which discloses the adsorption of oxygen and nitrogen from an argon-rich feed taken from the rectification column of a cryogenic air separation plant. According to this patent, argon of high purity is separated and recovered from a crude argon stream containing minor amounts of oxygen and nitrogen, by selective adsorption of these contaminants in a series of adsorption columns (beds). In the preferred embodiment, the system utilizes two separate adsorbent columns in series wherein the first column contains a nitrogen equilibrium selective adsorbent (e.g. zeolite) that is used for nitrogen removal, and the second bed contains an oxygen rate selective adsorbent (e.g. carbon molecular sieve) used for oxygen removal. Further purification of the recovered argon is carried out by catalytic hydrogenation of residual oxygen therein.
Many more variations of the original PSA cycle can be found in the literature. For example, U.S. Pat. No. 5,346,536 by Kaneshige et al., describes PSA processes for N2 production, wherein, top-to-top and bottom-to-bottom bed equalization steps are included in the PSA cycles, and carbon molecular sieve (CMS) is used as the adsorbent. The inclusion of the top-top and bottom-bottom equalization steps in the PSA cycle result in enhanced N2 purity and the minimization of CMS pulverization. In addition, McCombs et al., U.S. Pat No. 4,263,018, proposes the use of uninterrupted feed during bed-bed equalization step(s). For example, according to McCombs et al., uninterrupted feed to the PSA process is conducted simultaneously during the bottom-to-bottom bed equalization step.
All of the aforementioned processes have high capital costs and/or consume a lot of energy. Consequently, there is a need to provide a highly efficient process to recover high purity argon in addition to oxygen and nitrogen from feed air.
This invention relates to a vacuum pressure swing adsorption (VPSA) process to be used for argon purification to obtain high purity ( greater than 99.999%) argon. The VPSA process described herein is for integration with a cryogenic air separation plant wherein crude argon from the cryogenic plant is fed continuously to the VPSA process for argon purification. The integration of the VPSA process with the cryogenic plant has several important advantages over the three stage cryogenic processes that are described for example in U.S. Pat. No. 5,440,884 and references therein, for producing high purity argon. For example, due to the small difference in the relative volatility between argon and oxygen, a large or superstaged argon column is required to produce high purity argon using a three stage cryogenic process. However, by integrating a VPSA process in the cryogenic process to produce high purity ( greater than 99.999%) argon, the number of equilibrium stages in the argon side arm column required to get a similar argon recovery is significantly reduced, resulting in a considerable reduction in capital cost associated with superstaging the side arm column to produce high purity argon.
Another advantage of the hybrid VPSA/cryo process is that no post purification is required to obtain high purity argon. For example, in prior art processes using hydrogen deoxo in post purification processes, the impurity (oxygen) contained in the crude argon stream, may be removed by integrating the cryogenic process with the deoxo process. Thus, the oxygen contained in the crude argon stream is allowed to react with excess hydrogen to produce water. Inherent in the use of hydrogen for the deoxygenation process is the contamination of the desired product (argon) with water and the unreacted species (e.g. hydrogen). Consequently, additional purification unit(s) may be required after the deoxygenation process. In addition, since large quantities of hydrogen are typically required for the desired purification, this hydrogen deoxo process could become too costly and obsolete in those parts of the world where hydrogen availability and shipment are limited. Also, note that most of the aforementioned disadvantages also apply to O2 getter deoxo processes.
Another advantage of this new hybrid VPSA/cryo process over prior art hybrid PSA/cryo processes, such as disclosed in U.S. Pat. No. 4,477,265, is the novel steps in the VPSA cycle. For example, the two bed VPSA process of this invention has no feed interruption during bed-to-bed equalization. According to this invention, during the bottom-to-bottom bed equalization step, the bed that is rising in pressure is also receiving feed gas simultaneously. In addition, if nitrogen is present in the crude argon, then the removal of N2 is achieved using a layer of N2 equilibrium selective adsorbent, and a second layer of adsorbent, preferably, an oxygen selective adsorbent such as carbon molecular sieve (CMS), is placed on top of the N2 equilibrium selective adsorbent to perform the argon-oxygen separation. The arrangement of layers of adsorbents in the same vessel for N2 and O2 removal from crude argon differs from the two beds in series as disclosed in U.S Pat. No. 4,477,265. Also, in the preferred mode of operation of this invention, the VPSA process used an oxygen rate selective adsorbent such as CMS to separate an argon-oxygen mixture. In addition, in the preferred mode of operation, the novel cycle of this invention uses countercurrent evacuation before and after the purge step to achieve high purity argon at high yield. Note that in the aforementioned prior art processes, the purging step follows countercurrent evacuation, then pressurization follows the purge step. We have found that evacuating the bed after purging with argon countercurrently results in a bed with lower adsorbed phase loading at the end of evacuation, i.e., lower residual loading on the bed after regeneration, thus, lower adsorbent requirement, and a more efficient process. Also, according to this invention, the two bed VPSA process does not require any storage vessel to store feed gas (crude argon) during bed-to-bed equalization step, thus, lower capital cost. The bed-to-bed equalization step is very important to minimize bed fluidization, CMS pulverization, and to achieve enhanced argon recovery. Since a continuous feed (crude argon) comes from the cryogenic air separation plant, the two bed PSA process is designed to handle a continuous feed during bed-bed equalization. However, in prior art processes, except for U.S. Pat No. 4,263,018, during bed-to-bed equalization, the feed to the PSA process is interrupted, or more than two beds are used, or additional storage tanks are required to handle continuous feed. In addition, although prior art hybrid PSA/cryo processes used oxygen rate selective adsorbents such as 4A zeolite (U.S. Pat. No. 4,239,509) or carbon molecular sieves (U.S. Pat. No. 4,477,265) the novel steps in the PSA column cycle of this invention differ significantly from the prior art processes.
Finally, in the preferred mode of operation of this new VPSA/cryo process, a hydrostatic head pressure could be used to compress the crude argon stream from the cryogenic process to the high pressure required by the VPSA process. Thus, most or all of the energy for feed compression comes from the hydrostatic head pressure. Since the need for a crude argon compressor is eliminated, additional cost reduction is achieved in the production of high purity argon. In addition, the argon usually lost in the waste stream of the VPSA process is substantially recovered by recycling the gas to the cryogenic unit.